1. Field of the Invention
The present invention relates to integrated circuit devices and integrated circuit device manufacturing, including manufacturing of non-volatile memory devices, and more particularly to devices and processes for protection of integrated circuitry from plasma damage during manufacture.
2. Description of Related Art
In the manufacturing of integrated circuits, the processes are utilized which utilize activated ions. For example, backend processes including metal etching, photoresist stripping, and deposition of inter-metal dielectrics, involve plasmas which induce charge on the structures being treated. These plasma induced charges may damage underlying structures in the device, including structures critical to device performance. For example, tunnel dielectrics used in flash memory devices, gate dielectrics, and inter-polysilicon dielectrics, can be damaged by plasma induced charge. Furthermore, the charge storage structure utilized in SONOS cells is particularly susceptible to damage by plasma based processes. SONOS memory devices are described in U.S. Pat. No. 6,011,725.
One characteristic of the plasma induced charge is that it may be either positive or negative, and different types of damage can occur based on the type of induced charge.
In conventional semiconductor devices, protection from plasma induced charge is provided by forming a protection diode as shown in FIGS. 1A and 1B. A protection diode between a p-type region and an n-type well is formed to release “positive” charge from a node coupled to the p-type region as illustrated in FIG. 1A. A protection diode between an n-type region and the p-type well is formed to release “negative” charge from a node coupled to the n-type region, as shown in FIG. 1B. However, the structure of FIG. 1A does not discharge negative charge until the charge buildup is negative enough for junction breakdown. Likewise, the structure of FIG. 1B does not discharge positive charge until the charge buildup is positive enough for junction breakdown. These protection devices are useful and easy to implement. However, if the plasma exposure is too high, damage continues to happen with these prior art structures.
In floating gate memory devices as shown in FIG. 2, or other stacked gate structures, plasma charge builds up in the floating gate structure, and results in the cell threshold shifting up during the manufacturing process. Typically, the device is exposed to ultraviolet radiation after processing. The radiation causes the built up charge to be discharged to near normal conditions. However, damage to the cell structure caused by the plasma processes is not repaired.
For SONOS devices as illustrated in FIG. 3, plasma damage is more difficult to repair. The charge buildup caused by plasma based processes cannot be neutralized by ultraviolet exposure, as used in floating gate nonvolatile memory cells. The ultraviolet radiation injects additional electrons into the nitride film shifting the threshold voltage of the memory cell. Furthermore, because the threshold voltage is increased in SONOS devices for either positive or negative charge stress, the typical junction breakdown protection device is not sufficient. Back-to-back junction diodes can be used. However, the combination does not provide sufficient protection because the cell will be damaged before the junction breakdown levels are reached.
Accordingly, it is desirable to provide a protection circuit for use in the manufacturing of integrated circuits that protect both positive and negative charge damage. Furthermore, the protection circuit should not affect device operation after manufacturing. Finally, the protection circuit should be easily manufacturable.